docs: upate contributing guide to be more accurate

[aghArdeshir]

Spotted two small "imperfections" in CONTRIBUTING.md guide while reading them.

What I did

• git remote add raises and error: remote upstream already exists. That's because GitHub adds them for forked repositories.

• npm install had an explanation, but npm run build did not.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 8d741f5f734fdb81abe1cc73b9862b997c5fe975

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[aghArdeshir]

(done :heavy_check_mark:) ~~Trying to edit my commit message to match the conventional commit format ... (for some reason, git/GitHub can't find my fork/branch)~~